Heart failure is a common form of heart disease affecting nearly 6 million Americans. Despite recent advances in therapy, heart failure is associated with a high risk for hospitalization and death. Autonomic dysregulation of the cardiovascular system, characterized by heightened sympathetic activity and withdrawal of parasympathetic activity promotes progression of heart failure. Pharmacological blockade of sympathetic overactivity is associated with reduced mortality risk, but there are few data on pharmacologic augmentation of parasympathetic withdrawal. Acetylcholinesterase inhibitors augment parasympathetic neurotransmission by blocking the enzymatic breakdown of acetylcholine at cholinergic receptor sites. Pyridostigmine is a short-acting, reversible acetylcholinesterase inhibitor approved by the FDA for the treatment of myasthenia gravis. We now propose a Phase II prospective randomized, double-blind trial to compare 12 weeks of treatment with ascending doses of pyridostigmine (15, 30, and 60 mg every 8 hours) vs. matching placebo in 60 patients with symptomatic chronic heart failure associated with left ventricular systolic dysfunction. The clinical pharmacology of pyridostigmine will be investigated for each of the following specific aims: 1) To characterize the effects of oral pyridostigmine vs. placebo on sympathovagal balance in patients with chronic heart failure; 2) To characterize the safety and tolerability of oral pyridostigmine vs. placebo in patients with chronic heart failure; and 3) To characterize the steady state pharmacokinetic and pharmacodynamic properties of repeated oral dosing of pyridostigmine in patients with chronic heart failure. Mixed effects models will be used to determine the association between study drug assignment and physiological markers of sympathovagal balance (post-exercise heart rate recovery, heart rate variability, cardiovagal baroreceptor function, and rest/exercise blood catecholamine levels), descriptive statistics to characterize safety/tolerability measures (exercise capacity, quality of life, biomarkers of disease progression, cholinergic symptoms score), and population pharmacokinetic/pharmacodynamic modeling to characterize the relationship between study dosing, study drug blood levels, the degree of cholinesterase inhibition and the measures of sympathovagal balance and safety/tolerability. The overall goal is to further characterize the potential of pyridostigmine as a novel treatment in heart failure subjects and obtain information necessary to evaluate the feasibility/logistics of a future Phase III outcomes study in heart failure patients. The proposed studies will provide new data that are critically needed to direct the future development of this promising drug as a novel therapeutic approach for reduction of morbidity and mortality in heart failure patients.